PTH-dependent regulation of renal phosphate homeostasis remains incompletely understood. PTH downregulates the activity of two kidney-specific sodium-phosphate co-transporters, NPT2a and NPT2c. NPT2a handles 70-80% of phosphate reabsorption and it is regulated through PTH-dependent mechanisms that involve cAMP/PKA signaling down-stream of the PTH/PTHrP receptor (PTHR1). Consistent with such an important role of the Gs_/cAMP/PKA pathway in renal phosphate handling, patients affected by some forms of pseudohypoparathyroidism (PHP) develop PTH-resistant hyperphosphatemia because of impaired Gs_ expression or function. NPT2c appears to be regulated through PLC/PKC-signaling mediated by the PTHR1. NPT2c handles only 20-30% of renal phosphate reabsorption, yet inactivating homozygous/compound heterozygous mutations lead to hereditary hypophosphatemic rickets with hypercalciuria (HHRH), an autosomal recessive disorder. NPT2a and NPT2c thus are likely to have non-redundant roles in renal phosphate homeostasis. The PTHR1 is expressed at the baso-lateral membrane (BLM) of renal proximal tubular cells where it regulates NPT2a via the cAMP/PKA pathway; it is, however, unknown whether IP3/PKC-dependent signaling of PTHR1 at the BLM is involved in the regulation of NPT2a and/or NPT2c expression. At the brush-border membrane (BBM), the PTHR1 appears to activate only cAMP/PKAindependent signaling pathways, and evidence from our preliminary studies suggest that these actions regulate NPT2c expression. To elucidate the mechanisms underlying the PTH-dependent regulation of NPT2a and NPT2c, we propose to use mice expressing a PTHR1 mutant that is deficient in PLC-signaling, mice that are null for NPT2a, and mice that lack Gs_ expression in the proximal renal tubules (Aim I). Furthermore, we will use genetically-modified proximal tubular LLC-PK1 cells that show PTH-dependent down-regulation of phosphate transport when expressing the PTHR1, in combination with either NPT2a or NPT2c (Aims II and III). Our novel LLC-PK1 cell model will be manipulated to express different wild-type and mutant proteins and treated with PTH and signal-selective PTH analogs applied to either apical or baso-lateral surfaces, thus presenting a valuable tool for our investigations. We predict that the combination of these in vivo and in vitro approaches will provide important new insights into the regulation of phosphate homeostasis and are likely to lead to improved treatment options for human disorders associated with impaired renal phosphate excretion, including hypoparathyroidism, tumoral calcinosis, PHP, and particularly, chronic kidney disease.